What is often missing in air brake equipped trailers made before 1975?

In air brake equipped trailers made before 1975, spring brakes were often missing. Spring brakes are a crucial safety feature that allows vehicles to come to a stop safely in the event of a brake system failure. They work by using large springs to apply the brakes when air pressure is released, providing a reliable means to slow down or stop the vehicle under emergency conditions.

Older trailers typically utilized a system that only relied on hydraulic brakes or standard air brakes without the backup provided by spring brakes. This created potential safety hazards, as there was less assurance that the vehicle could be brought to a stop effectively if the primary braking system failed. The addition of spring brakes to trailers has become standard practice to enhance safety, particularly in combination vehicles.